1. Field of the Invention
This invention relates to a magnetoresistance device capable of reading a small magnetic field strength change as a greater electrical resistance change signal, a magnetic multilayer film suitable for use therein, and a method for preparing the magnetic multilayer film. The term "magneto-resistance" is often abbreviated as MR, hereinafter.
2. Prior Art
There are growing demands for increased sensitivity of magnetic sensors and increased density of magnetic recording. Researchers strive for the development of magnetoresistance effect type magnetic sensors (simply referred to as MR sensors, hereinafter) and magneto-resistance effect type magnetic heads (simply referred to as MR heads, hereinafter). Both MR sensors and MR heads are MR devices for measuring the strength of a magnetic field applied thereacross by utilizing the principle that a reading sensor portion of MR material changes its electric resistance in response to an external magnetic field. The MR material detects the strength of an external magnetic field itself. Unlike inductive magnetic heads, the MR heads produce outputs which do not depend on their speed relative to magnetic recording media, ensuring high outputs upon reading of high density magnetic recording signals. The MR sensors have the advantage of high sensitivity.
Conventional MR heads using magnetic materials such as Ni.sub.0.8 Fe.sub.0.2 (Permalloy) and NiCo as MR material have less enough sensitivity to read ultrahigh density record of the order of several GBPI since their percent MR ratios are low.
Attention is now paid to artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked since their behavior is different from bulk metal. One of such artificial superlattices is a magnetic multilayer film having ferromagnetic metal thin films and non-magnetic metal thin films alternately deposited on a substrate. Heretofore known are magnetic multilayer films of iron-chromium and cobalt-copper types. Among them, the iron-chromium (Fe/Cr) type was reported to exhibit a MR ratio in excess of 40% at cryogenic temperature (4.2K) (see Phys. Rev. Lett., Vol. 61, page 2472, 1988). This artificial superlattice magnetic multilayer film, however, is not commercially applicable as such because the external magnetic field at which the MR ratio becomes maximum (that is, operating magnetic field strength) is as high as ten to several tens of kilooersted (kOe). Additionally, there have been proposed artificial superlattice magnetic multilayer films of Co/Ag, which require too high operating magnetic field strength.
Under these circumstances, a ternary artificial superlattice magnetic multilayer film having two types of magnetic layers having different coercive forces deposited with a non-magnetic layer interposed therebetween was proposed as exhibiting a giant MR change due to induced ferrimagnetism. Regarding such magnetic multilayer films, the following articles and patents are known.
(a) T. Shinto and H. Yamamoto, Journal of the Physical Society of Japan, Vol. 59 (1990), page 3061.
Described is a magnetic multilayer film of [Cu(x)--Co(y)--Cu(x)--NiFe(z)]xN wherein x, y and z represent the thickness in angstrom of the associated layer and N is the number of recurring units of Cu--Co--Cu--NiFe (the same applies hereinafter) wherein (x, y, z, N)=(50, 30, 30, 15). It produced an MR ratio of 9.9% at a maximum applied magnetic field of 3 kOe and about 8.5% at 500 Oe.
(b) H. Yamamoto, Y. Okuyama, H. Dohnomae and T. Shinjo, Journal of Magnetism and Magnetic Materials, Vol. 99 (1991), page 243
In addition to (a), this article discusses the results of structural analysis, changes with temperature of MR ratio and resistivity, changes with the angle of external magnetic field, a minor loop of MR curve, dependency on stacking number, dependency on Cu layer thickness, and changes of magnetization curve.
(c) U.S. Pat. No. 4,949,039 or JP-A 61572/1990
Ferromagnetic layers stacked alternately with non-magnetic intermediate layers align anti-parallel, exhibiting great magnetoresistance effect. A structure wherein an antiferromagnetic material is disposed adjacent one of the ferromagnetic layers is also disclosed.
(d) U.S. Pat. No. 5,315,282, EP 0 483 373 A1 or JP-A 218982/1992
Disclosed is a magnetic multilayer film having two types of magnetic layers having different coercive forces stacked through an intervening non-magnetic layer. An exemplary structure includes a Ni-Fe layer of 25 .ANG. or 30 .ANG. thick, an intervening Cu layer, and a Co layer of 25 .ANG. or 30 .ANG. thick.
(e) JP-A 122963/1994
Disclosed is a magnetic multilayer film having two types of magnetic layers having different coercive forces stacked through an intervening non-magnetic layer. By controlling the squareness ratio of the two magnetic layers, the slope of an MR curve at zero magnetic field is increased and the MR effect at high frequency is improved.
As compared with Fe/Cr, Co/Cu and Co/Ag, these field-induced ferrimagnetic multilayer films are inferior in the magnitude of MR ratio, but experience a rapid change of MR ratio under an applied magnetic field of less than several tens of oersted. They are thus effective MR head materials coping with a recording density of about 1 to 100 Gbits per square inch. While MR heads are required to operate under a magnetic field at a high frequency of at least 1 MHz for high density writing/reading, the magnetic multilayer film described in (e) is fully practical due to its improved MR effect at high frequency.
For increased sensitivity, the MR head has the so-called shielded structure wherein a magnetic multilayer film or Permalloy serving as a magneto-sensitive section is interposed between a pair of soft magnetic layers with an non-magnetic layer interposed therebetween. The distance between the pair of soft magnetic layers, known as a shield length, is very important. The shield length must be reduced as the recording density increases. However, in conventional MR heads using Permalloy in the magneto-sensitive section, the magneto-sensitive section has an increased total thickness because a shunt layer and a soft film bias layer are added to the Permalloy. This prevents the shield length from being reduced, leaving a problem. In MR heads using a magnetic multilayer film having a great slope of a MR curve at zero magnetic field as described in (e), the shield length cannot be reduced if there is a large number of recurring units.
The shield length may be reduced by reducing the number of recurring units N in a multilayer structure. In the above-referred article (b), Journal of Magnetism and Magnetic Materials, Vol. 99 (1991), pp. 243-252, FIG. 9 is a diagram of MR ratio at RT as a function of stacking number in a multilayer, which contains some examples wherein the number of recurring units N is 3 or less. In these examples, however, the percent MR ratio is as low as 4% at N=3 and 1.2% at N=1. As seen from these examples, prior art field-induced ferrimagnetic multilayer films have the problem that if the number of recurring units N is reduced in order to reduce the shield length, the percent MR ratio is concomitantly reduced. Even with the magnetic multilayer films described in (e), where the number of recurring units N is small, it is difficult to provide satisfactory MR characteristics and the slope of a MR curve is reduced especially under a high frequency magnetic field.
Since a complex laminate structure is used in MR heads or the like, patterning and flattening steps require heat treatment such as baking and curing of resist material, which in turn, requires heat resistance at temperatures of about 300.degree. C. However, prior art artificial superlattice magnetic multilayer films tend to deteriorate by such heat treatment, and lose heat resistance especially when the number of recurring units N is small.